The number of the SMS center is +32456002000.
iPhone users can set the SMS center by entering the code *5005*7672*+32456002000# and pressing the call button. Hold the 0 for a few seconds to make the + sign. Doesn't work? Add an extra * at the start of the code!
For all other operating systems, users can enter the number of the SMS center in the settings of their device (usually in the settings of your text app). Our configuration guides can help you figure this out. :-)